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We have established that the transformer hum is caused by the extension and contraction of the core laminations when magnetized. Under alternating fluxes, we can expect this extension and contraction to take place twice during a normal voltage or current cycle. This means that the transformer is vibrating at twice the frequency of the supply, i.e. for 60 cycles per second supply frequency, the noise or vibration is moving at 120 cycles per second. This is called the fundamental noise frequency.
Nothing is this world is ever perfect and so it is with transformer cores. Since the core is not symmetrical and the magnetic effects do not behave in a simple way, the resultant noise is not pure in tone. That is the noise or vibration produced is not only composed of a 20c/s frequency, we find from practical work that transformer noise is made up of frequencies of odd multiples of the fundamental known as 1st, 3rd, 5th and 7th harmonics. |
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This means we get noise frequencies of 120 (1st), 360 (3rd), 600 (5th), 840 (7th) cycles per second. They are not equally important for we find that the first and third harmonics predominate and produce most of the transformer sound. |
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| It is important to know this because, with this knowledge, we can measure the amount of noise at these frequencies and determine whether amongst a number of other noises, we really are picking up a transformer noise. |
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